1. Field of the Invention
The present invention relates to a device and a method for monitoring the body movements of a patient on a support.
2. Description of the Prior Art
Patients who spend long periods of time in bed are known to be at risk from developing pressure sores (tissue necrosis). To prevent this, regular turning of the patient is required, or a pressure relieving mattress, such as the Airwave.TM. sold by the applicants, may be used. The Airwave mattress is of the type illustrated in GB-A-1595417.
However in order to decide which form of mattress or what turning regime is required, it is necessary to assess the risk of individual patients for developing pressure sores. Studies carried out in 1961 by Exton-Smith and Sherwin (The Lancet, Nov. 18, 1961, pages 1124-1127) came to the conclusion that the risk of developing pressure sores is inversely related to the number or rate of spontaneous body movements. Exton-Smith and Sherwin counted body movements of patients using an inertial switch connected via a ratchet device to the mattress.
Attempts have been made to develop methods of monitoring spontaneous body movements. Watching is clearly not economically practical. Most efforts to date have been aimed at providing devices for attachment to legs of a bed frame which sense changes in the load on each leg of the bed frame, and/or changes in the centre of gravity of the bed frame. These changes can be related to patient movement, but the devices are bulky, expensive and difficult to install and remove from the bed.
Proposals have also been made for use of inflatable bodies on which a person is lying, in order to monitor body movement. Apnoea detection alarms for infants are proposed in U.S. Pat. No. 5,515,865, GB-A-1261357 and U.S. Pat. No. 3,727,606 which seek to monitor breathing by detecting pressure fluctuations within a mattress. U.S. Pat. No. 5,515,865 proposes among other things a pressure monitor connected to a water chamber. In GB-A-1261357 a heated wire sensor detects flows between mattress cells. In U.S. Pat. No. 3,727,606 a pressure sensor is attached to the mattress and is protected from large pressure changes caused by the infant's gross movements by a pop-off valve. These devices provide an alarm signal if no breathing movement is detected in a certain time period.
GB-A-2199953 discloses a device for measuring pressure at the interface between a patient's skin and a mattress, such as an alternating pressure mattress, to study the performance of the mattress. A small inflatable cell is placed at the interface location to be measured, and is periodically inflated by means of a compressor. The cell contains electrical contacts which break and make as the cell is inflated and deflated. By this method the interface pressure variation is plotted.
WO 91/13575 describes a mattress which monitors and records movements of a person lying on it. The mattress has many inflated cells in square arrays in four zones, at waist, hip, shoulder and foot, and a pressure-responsive readout device for each zone which reads out and records pressure changes resulting from movement of the patient as charts. It is suggested that from the charts of pressure fluctuations at these four zones, a doctor can diagnose sleep disorders, and it is described how in the charts breathing movements and arm and leg movements can be detected, within time zones in which the patient was lying on their back, side and front. The device is complex, and provides only a display of pressure changes as an output.
None of those devices provide an output which is related to the risk of development of pressure sores by a patient.